Our present invention relates to a method of monitoring the contact burnoff of a tap changer and especially the burnoff of contacts which tend to arc in tap changers.
Tap changers have been used for a considerable time for the uninterrupted switching between taps of a tap transformer in electrical power distribution and such tapped transformers and their tap changers are utilized in large number throughout the world. The tap changer is utilized to select the transformer winding which is to be effective and has been designed to allow such switching under load. The tap changer for tap selection under load generally comprises switching contacts and resistance contacts. The switching contacts can directly connect the particular tap and section of the transformer winding, with the lines running to the load. The resistance contacts are briefly connected in circuit and bridge resistance into the circuit to allow uninterrupted tap selection under load. In recent years the tap changer could be equipped with thyristors (electronic switch devices) and vacuum switch cells as the switching elements but by far the greatest number of tap changers in use today and in the near future utilize mechanical contacts which are subject to burn off by the switching arc which may be formed.
To minimize the contact burnoff, the switching contacts and the resistance contacts can be composed of arc-resistant copper-tungsten alloys. Nevertheless, upon switchover of the contacts arcs are generated which can melt small quantities of the contact material and cause burnoff and evaporation of some of the contact material. The result is a contact burnoff which is an important criterium in the maintenance and operation of a tap changer. The contacts in the past have been frequently inspected and determinations as to burnoff have been made. The burnoff in the switching and resistance contacts is a significant consideration in the operation of the tap changer. If the contacts burn off at different rates, the switching and overlapping or bridging intervals of the individual switching steps can vary within the sequence in a tap changing operation so that the tap changer if the contacts burn off at different rates, the switching and overlapping or bridging intervals of the individual switching function can become unreliable. In general, the burnoff will have a maximum permissible burnoff difference or burnoff limit. If these values are exceeded the contacts must be replaced by new contacts or the resistance contacts and the switching contacts must be interchanged. When contacts are completely burned off, they must be replaced immediately.
There are numerous processes available for contact burnoff or contact wear monitoring whereby the residual life of a contact or some other similar factor can be reviewed for switching contacts and tap changers or other high voltage switching contacts. These can be considered in different groups.
For example, DE-GM 296 19 365 and EP 0 948 006 provide a purely optical process for determining residual life or burnoff state.
DE-OS 35 15 027 and DE-PS 40 28 721 describe processes in which the arc current between the contacts is determined and is used as a criterium for the burnoff.
DE-PS 195 44 926 describes a process in which the arc voltage is used.
DE-OS 44 27 006 describes a process in which the contact pressure of the switching element is utilized as a criterium of contact burnoff. WO 97/28549 describes a process for monitoring the switch movements, i.e. the timed sequence in tap selection or tap changing.
In WO 96/13732 a process has been described for monitoring the insulation breakdown criterium for a switch contact subject to wear, utilizing an additional signal line.
Japanese open application Hei-4-64206 describes a process utilizing a calculation which is a function of the number of switchovers carried out by a tap changer.
Reference may also be had to DE 195 30 776 C1 which discloses a process for monitoring a tap changer operable under load whereby during the switching under load, the arc which is formed is detected from time to time and by comparison of the times between the individual arcs or by comparison of the durations of the length of the individual arc with respect setpoint values as characteristic values for the tap changer, a determination of the contact burnoff can be obtained. The determination is indirect and when the life of the contacts is exceeded, i.e. the burnoff has progressed beyond a permissible limit, the replacement can be undertaken. A direct detection of burnoff or monitoring is not however possible.
DE-OS 27 27 378 describes a device for monitoring operation of a tap changer in a general way in which the burnoff is determined by a load current measurement utilizing a current converter. In many cases this system is not suitable for certain tap changers.
By and large the processes described above have not found widespread use with tap changers for a variety of reasons. Direct optical and mechanical techniques are not practical because of the location of the contacts to be monitored in the interior of the tap changer, i.e. usually in an oil bath. Processes which require additional measuring conductors to run into the region of the contacts in the tap chamber are also not suitable since the passage of these converters through the tap changer wall reduces the breakdown voltage of the housing and the system. Processes which utilize the arc current, the arc voltage or the number of switching operations have generally been found to be insufficiently reliable.
It is therefore the principal object of the present invention to provide an improved process or method for monitoring contact burnoff in tap changers which can ensure in a reliable and simple manner a substantially exact measurement of the burnoff of the contact without requiring visual examination or direct measurements at the respective contacts and which can generate an output upon a contact burnoff exceeding a predetermined degree.
Another object of the invention is to provide a burnoff monitoring method which is free from the drawbacks of the prior art system mentioned previously and which does not adversely affect breakdown voltage of the tap changer.
It is also an object of this invention to provide a method of monitoring contact burnoff in the switching contacts and resistance contacts of a tap changer wherein the contacts which tend to arc can be submerged in oil.
These objects are attained, in accordance with the invention in a method of monitoring contact burnoff in a tap changer for a transformer having a multiplicity of taps, the method comprising the steps of:
(a) storing values of respective nominal tap voltages (US), a limiting value for the permissible contact burnoff for switching contacts and resistance contacts of the tap changer, and tap-changer-specific parameters a, b and k;
(b) detecting a current tap setting of the tap changer;
(c) incrementing an index n with each tap change by
(c1) stepping the tap changer to a selected tap,
(c2) measuring a respective load current (JL) of the selected tap, and
(c3) reading out the permanently stored values for the nominal tap voltage (US) of the selected tap;
(d) calculating a switching current (JSK) of the respective switching contacts and a switching current (JWK) of the respective resistance contacts in accordance with the relationships:             J      SK        =                  J        L            ParSec                  J      WK        =                            U          S                +                              J            L                    ·                                    R              0                                      s              res                                                  2        ·                  R          0                    
wherein ParSec is a number of parallel sectors, R0 is a magnitude of a bridging resistance of the tap changer for the selected tap and sres is a resulting current splitting;
(e) calculating the respective burnoff rates Ask of the respective switching contacts and Awk of the respective resistance contacts from the relationships:             A      SK        =          a      ·              J        SK        b                        A      WK        =          a      ·              J        WK        b            
(f) summing up the burnoff rates (Ask) and (Awk) to obtain total volume burnoffs (GASKn)for the switching contacts and GAwkn 
for the resistance contacts by the relationships:
GAwkn=GAwknxe2x88x921+Awk; 
GAskn=GAsknxe2x88x921+Ask; 
(g) calculating the respective burnoffs in millimeters of contact thickness for the switching contacts. GAdskn and for the resistance contacts GAdwkn over the respective contact areas F by the relationships:             GAd      sk      n        =                            GA          sk          n                F            ·      k                          GAd        wk        n            =                                    GA            wk            n                    F                ·        k              ;          xe2x80x83        ⁢    and  
(h) comparing the values G{overscore (A)}dskn and GAdwkn with the permanently stored limits and generating a report upon overstepping of the permanently stored limit or a percentage thereof.
According to a feature of the invention the respective burnoff rates (Ask) and (Awk) are obtained from the calculated switching currents (JSK) and (JWK) in accordance with the relationships:                     A        sk            =              a        ·                  J          sk          b                ·        s              ,          xe2x80x83        ⁢    and                      A        wk            =              a        ·                  J          wk          b                ·        s              ,  
where s is a safety margin.
The actual contact burnoff is measured after a large number of switchings and the corresponding actual volumetric contact burnoff is calculated to obtain a factor f by the relationship:             volumetric      ⁢              xe2x80x83            ⁢              burnoff        measured                    cumulative      ⁢              xe2x80x83            ⁢      volumetric      ⁢              xe2x80x83            ⁢              burnoff        calculated              =  f
and
each respective burnoff rate is corrected in accordance with the relationship:
Anew=ƒxc2x7Aold, 
whereby the respective corrected value (Anew) is then used for future calculations in the method.
The invention thus provides a system for determining the contact burnoff state of each contact from a respective burnoff rate A. The process steps are carried out, in accordance with the invention in a computer in which the characteristic parameters of the respective tap changer, whose contacts are to be monitored, are stored in a nonvolatile manner together with the burnoff limits, the exceeding of which results in a warning or other signal generation or alert.
As has already been indicated, the contact burnoff of the respective switching contact or resistance contact is determined in the volume unit of the contact material which is lost, for example in mm3 from the specific burnoff rate. This burnoff rate A with the physical unit mm3/switching operation, i.e. the volume unit per switching operation, is a parameter which is a function of the material from which the contact is constituted and the current carried by the contact. The burnoff rate is thus given by the relationship:       A    ⁡          [                        mm          3                          switch          ⁢                      xe2x80x83                    ⁢          operation                    ]        =      a    ·          J      b      
In this relationship J is a current which is switched by the respective tap changer. It is determined by the computer in a known manner from the actual load current of the transformer which is measured, the true voltage step between two neighboring winding taps between which the switchover is to be made and the configuration of the tap changer. The values a and b are tap- changer specific parameters which have been stored in a nonvolatile manner in the memory of the computer. The factor a lies in the range of 10xe2x88x925 to 10xe2x88x922. For a time M tap changer as manufactured by Maschinenfabrik Reinhausen GmbH of Regensburg, Germany, a is preferably 8.5xc2x710xe2x88x925. The value of b is in the range of 0.8 to 2.2. For the aforementioned type M tap changer b is preferably 1.16.
The determination of the burnoff rate should be obtained within a tolerance band which permits reliable response by the user. It has been found that the contact burnoff is affected by certain unpredictable and difficult to calculate influences which can give rise to significant fluctuations. As a consequence in the determination of the burnoff rate, a safety factor s is introduced which can be of an amount 10 to 12%. This has been found to be sufficient to cover the variations which can arise in practice. Thus according to a feature of the invention the burnoff rate can be obtained from the following relationship:       A    ⁡          [                        mm          3                          switch          ⁢                      xe2x80x83                    ⁢          operation                    ]        =      a    ·          J      b        ·    s  
In this manner the burnoff rate is obtained with the built-in safety factor.
It is possible in accordance with the invention to increase the precision of the determination of the burnoff rate still further by eliminating the flat rate approach with the safety factor previously described by iteratively determining the burnoff rate. In that case, the actual contact burnoff is measured after a representative number of switching operations. This can be carried out in the framework of routine inspection. From the measured values, the actual volume burnoff per contact is obtained and compared with the calculated volume burnoff to provide the correction factor f previously described. In that case, the calculation utilizes the following relationship:       A    ⁡          [                        mm          3                          switching          ⁢                      xe2x80x83                    ⁢          operation                    ]        =      f    ·    a    ·          J      b        ·    s  
The computer determination of the burnoff rate A according to the invention is integrated in a method of monitoring the contact burnoff. The process of the invention thus not only covers the calculation of the burnoff rate A but also the subsequent determination of the cumulative contact burnoff at each respective switching contact as well as the generation of any warning or other signal which is required by the situation.
A special advantage of the invention is that the monitoring of the contact burnoff of the contacts in the tap changer can be carried out in a simple manner without the need for access to the contacts themselves to view or measure them in any way. A further advantage of the invention is that the invention can be implemented in a complex tap changer and/or transformer monitoring system directly. The process of the invention allows the need for replacement of the contacts to be reliably determined. It avoids premature contact replacement which may be unnecessary and costly, and also prevents delay in contact replacement when the latter is necessary and thereby avoids the interruptions in function and difficulty in the replacement when the same is necessary.